Fix #5271 Add note to extra-deps documentation

Author: mpilgrem

doc/yaml_configuration.md

@@ -208,6 +208,12 @@ extra-deps:
 the configuration means "extra-deps packages in directories
 `/dir1/dir2/my-package` and `/dir1/dir2/dir3/my-other-package`".
 
+!!! note
+
+    A local file path that has the format of a package identifier will be
+    interpreted as a reference to a package on Hackage. Prefix it with `./` to
+    avoid that confusion.
+
 !!! note
 
     A specified extra-dep that does not have the format of a valid Pantry